Cockpit: right-align the milestone label at the row's edge, no space reserved on rows without one

[MJohnson459]

Moves the cockpit's milestone label from a fixed 16-column column before the project to the right edge of the row. Before this change, rows with no milestone still lost 17 columns of title to a blank column, and most rows have no milestone.

milestones::row_line now builds both the queue row (action_row_line, which also covers the proposals under a folded-open digest) and the running-strip row. The caller passes the row's prefix, its title and its badges; draw_queue and draw_running pass the pane's inner width. On a row whose task belongs to a milestone, the label (the nearest milestone's title cut to 16 columns with an ellipsis, plus +N when the task has several) sits flush against the right edge, two spaces clear. The title is truncated with an ellipsis to fit, and the badges stay directly after it. When fewer than 20 columns of title would remain, the row drops the label. A row with no milestone reserves nothing and renders as it did before #191. Widths are counted in display columns through ratatui's Span::width, so no new dependency is needed. DESIGN.md §9 now describes this placement instead of the column.

Verification: new tests in ui/milestones.rs render the full frame through TestBackend. Each one checks both a queue row and a running-strip row for these cases:

• a row with no milestone at 100 columns matches the pre-Milestones in the TUI: a fifth tab, a cockpit column, browser grouping and a picker #191 layout
• two member rows end their labels against the pane border in the same column
• a 120-character title keeps the label whole and ends in …
• two milestones show Dock +1
• at 60 columns the label is dropped and the title runs to the edge

deep_flag_renders_in_queue_browser_and_detail goes back to its pre-#191 assertions. cargo fmt, cargo clippy --workspace --all-targets -D warnings and cargo test --workspace (956 tests) pass. I also printed the rendered frames at 100 and 60 columns during development and checked them by eye.